1. Field of the Invention
This invention relates to catalysts comprised of a particulate substrate with a composite sputtered thin film on the surface thereof the film comprising one or more metals having catalytic activity and a cosputterable support material.
Ideally a catalyst is a substance that directly accelerates a chemical reaction without itself undergoing either a physical or chemical change. Because of their unique catalytic properties and inherent stability, the metals: Pt, Pd, Rh, Ag, etc., or their alloys, are frequently used as catalysts. Usually it is too expensive to use these metals in bulk form, so that a small amount of metal is supported on a substrate such as SiO.sub.2 or Al.sub.2 O.sub.3. Another motivation for supporting the metal is to disperse it, so that for a given amount of catalytic material, the number of available catalytic sites for the chemical reaction to take place is increased.
2. Prior Art
Commonly used techniques to deposit metal catalysts on a support are impregnation and ion exchange. To obtain high dispersions, the support is typically porous and has a high surface area. In many instances, the support is not inert under reaction conditions and deleterious reactions can occur on its surface, causing loss in catalyst selectivity. Although high dispersions of the catalytically active phase can be obtained during catalyst preparation, it is often difficult to preserve this disposition under conditions in which the catalyst is used. In cases where the support surface causes loss in catalyst selectivity and where the support cannot help maintain the catalyst surface area under reactor conditions, a bulk metal catalyst can be used. The synthesis of formaldehyde by oxidative dehydrogenation of methanol is an example of a process which occurs at high temperature (.about.650.degree. C.) and uses a bulk Ag catalyst.
An ideal form for a catalyst would therefore be a highly dispersed metal on a low surface area, stable support. In G.B. No. 1,455,248 and U.S. Pat. No. 4,046,712 J. A. Cairns et al. disclose an attempt to make such a catalyst comprising a hard, substantially nonporous particulate substrate having a surface area not greater than about 20 sq. meters per gram and a sputtered deposit of catalytic material comprising one or more of platinum, rhodium, palladium, ruthenium, osmium and iridium. In G.B. No. 1,537,839, J. A. Cairns et al. disclose that it is sometimes beneficial to provide a partial covering of the support material on each sputter-coated support-particle. Preferably the partial covering is in the range of 5-20% of the available surface area. This partial covering may be carried out by contacting the sputter-coated support particles with a dispersion of powdered support material and subsequently firing to adhere the powder to support-particles.
S. K. Sharma and J. Spitz, Thin Solid Films 61, L13-15 (1979) have found that sputtered silver films exhibit hillock growth and agglomeration similar to that shown by silver and other metal evaporated thin films, [see, e.g., A. E. Presland et al., Prog. Surf. Sci. 3, 63 (1973)]. For example, quartz substrates covered with a 500 .ANG. sputtered film of Ag have 60% of the surface area free of Ag after 4 hours of annealing in oxygen at 372.degree. C. It is believed that hillock growth occurs as a result of the relaxation of the thermal stress developed during heating because of the different thermal expansion coefficient of the metal film and the substrate and that agglomeration results from surface diffusion with the kinetics determined by surface energy forces.
Sputtered composites of metals and insulators, known as cermets, have been used as electrical resistors for some time. [See e.g., the review article by B. Abeles et al., Adv. Phys. 24, 407 (1975) which also gives typical procedures used for cosputtering composites.]